The present invention relates to a high resolution optical system suitable for use in inspections and observations of micro pattern defects and foreign materials or the like typified in a semiconductor device manufacturing process and a flat panel display manufacturing process, and a pattern defect inspecting apparatus using the same.
With high integration of a semiconductor, circuit patterns show an increasing tendency to scale down. Under such a tendency, there has been an increasingly demand for a high resolution-based detection of pattern defects on a wafer to which masks and reticles used upon manufacturing a semiconductor device in a photolithography process, and circuit patterns formed on these, are transferred by exposure. As means for enhancing the resolution, may be mentioned a way to bring the wavelength of illumination light from visible light to ultraviolet light.
A mercury lamp has heretofore been used as a light source. Only required wavelengths were used by being optically selected from various emission lines held by the mercury lamp.
However, a problem arises in that each emission line of the mercury lamp is broad in light-emitting spectrum width and the correction of chromatic aberration of an optical system falls into difficulties, and a light source is scaled up to obtain a sufficient illumination intensity, thereby causing a decrease in efficiency and the like.
An exposure device equipped with a 248 nm-wavelength KrF excimer laser has recently been developed as a light source used for the exposure device upon semiconductor manufacture. However, the light source for the excimer laser is large in size and expensive. Further, a problem arises from the viewpoint of maintenance such as the need for predetermined safety measures because noxious fluorine gas is used.
As ultraviolet laser light sources, may be mentioned, for example, a laser device for wavelength-converting a solid YAG laser light beam by a non-linear optical crystal, an Arxe2x80x94Kr laser device, etc. Each of them can obtain ultraviolet laser light having a wavelength which ranges from 266 nm to 355 nm.
However, while these ultraviolet laser light sources respectively have the advantage of obtaining a high output as compared with a lamp used as a light source, they have coherence. When the ultraviolet laser light is illuminated to a circuit pattern, unnecessary coherent patterns (speckle noise) occur and hence they exert a bad influence on the inspection of the circuit pattern.
Increasing the resolution of a circuit pattern to be detected needs to set the magnitude of illumination light launched into an objective lens to a suitable magnitude and illuminate a specimen from various angular directions. Since, however, the laser is used as a point source of light, it encounters a difficulty in increasing an illumination angle.
Further, each of the ultraviolet laser light sources wavelength-converts the solid YAG laser light beam by means of the non-linear optical crystal to thereby obtain a third harmonic wave (355 nm) or a fourth harmonic wave (266 nm) of the YAG laser beam. A wavelength converting device for obtaining the UV laser light in this way has been known in each of Japanese Patent Application Laid-Open Nos. Hei 8-6082, Hei 7-15061, Hei 11-64902 and Hei 11-87814. However, when the crystal is irradiated with the laser for long hours, the interior of the crystal is deteriorated and hence the transmittance of the laser beam is significantly reduced. Therefore it is necessary to change a position to irradiate the crystal with the laser after the irradiation thereof for a predetermined time. As viewed from the standpoint of another aspect, Japanese Patent Application Laid-Open No. Hei 8-6082, for example, performs such feedback control that a mirror corresponding to some of a resonator is mounted to a piezoelectric device or the like and displaced to make frequency tuning inside the wavelength converting device, whereby a resonator length is varied to change a resonant frequency. The present publication describes that the fluctuation of the frequency takes place when such frequency tuning is detuned, so that the intensity of output light varies. The variation in the output light is not on the order of an ability to visually confirm it, and is a very momentary variation. However, this becomes a serious problem for an apparatus for inspecting a pattern defect at high speed.
In order to solve the foregoing problems, an object of the present invention is to provide a pattern defect inspecting apparatus capable of implementing micro patterns with high resolution and realizing a stable pattern defect inspection with a high degree of reliability with ultraviolet laser light as a light source.
In order to achieve the above object, the present invention provides a pattern defect inspecting apparatus, comprising a stage capable of placing a substrate to be inspected thereon and moving in X and Y directions; an ultraviolet laser light source for emitting ultraviolet laser light therefrom; an illumination optical system including a multispot shaper for forming a plurality of light-emitting images from the ultraviolet laser light emitted from the ultraviolet laser light source, a coherence reduction optical system for reducing coherence with respect to the ultraviolet laser light comprised of the plurality of light-emitting images formed by the multispot shaper, and a converging optical system for allowing a plurality of pieces of the ultraviolet laser light reduced in coherence through the coherence reduction optical system to converge on a pupil position of an objective lens, the illumination optical system applying the plurality of pieces of ultraviolet laser light focused on the pupil of the objective lens by the converging optical system onto the substrate to be inspected; a detection optical system including an image forming optical system for focusing an ultraviolet reflected light image from a circuit pattern formed on the substrate to be inspected, and an image sensor for receiving the ultraviolet reflected light image focused by the image forming optical system and converting the same into an image signal; an image signal processing circuit including an A/D converter for A/D converting the image signal obtained from the image sensor of the detection optical system, and a comparator for comparing a detected digital image signal converted by the A/D converter with a reference digital image signal to thereby detect a defect or a defect candidate in the circuit pattern and outputting inspection information in relation to the detected defect or the detected defect candidate; and an amount-of-light monitor unit for allowing some of the ultraviolet laser light emitted from the ultraviolet laser light source to branch off and detecting the intensity thereof.
The present invention also provides the pattern defect inspecting apparatus characterized in that laser speckles are suppressed by the coherence reduction optical system having such a configuration as to form a multi light-emitting image with the multispot shaper provided in an illumination optical path and two-dimensionally scan it on the pupil of the objective lens.
Further the present invention is characterized in that the ultraviolet laser light source comprises a laser device for emitting laser fundamental wave light therefrom, and a wavelength converting device for wavelength-converting the laser fundamental wave light emitted from the laser device into a double wave, both of which are placed in a clean container.
Furthermore the present invention is characterized in that the wavelength converting device comprises a resonator comprised of an optical member having high reflectance, and a non-linear optical crystal placed in a suitable position lying in an optical path of the resonator.
Still further the present invention is characterized in that the amount-of-light monitor unit comprises a division type optical device for measuring the amount of a displacement of an outgoing optical axis of the ultraviolet laser light source.
Still further the present invention is characterized in that the amount-of-light monitor unit monitors a variation in the amount of the ultraviolet laser light during inspection.
Still further the present invention also includes a central processing unit for detecting a malfunction on the basis of the intensity of the ultraviolet laser light detected by the amount-of-light monitor unit and storing the inspection information outputted from the comparator of the image signal processing circuit at a time at which the malfunction is detected, thereby enabling re-inspection.
Still further the present invention further includes a central processing unit for predicting the life of the ultraviolet laser light source and detecting a malfunction thereof, based on the variation in the amount of the ultraviolet laser light detected by the amount-of-light monitor unit to thereby determine whether the inspection should be carried out continuously.
Still further the present invention constitutes a system wherein when the output intensity of the ultraviolet laser light is always detected by the amount-of-light monitor unit and a change in the intensity thereof is greatly varied as compared with a constant reference value, positional coordinates of the stage at that time are stored and fed back to the result of inspection, and when an output value reaches a reference value or less even with respect to an output reduction in ultraviolet laser light with time, a crystal is moved to change a point to apply laser thereto.
Still further the present invention is characterized in that the coherence reduction optical system of the illumination optical system comprises a swinging first optical device or a rotating second optical device placed therein.
Still further the present invention is characterized in that the coherence reduction optical system of the illumination optical system includes a swinging first optical device and a rotating second optical device respectively placed in positions unconjugated with respect to each other.
Still further the present invention is characterized in that the ultraviolet laser light source is covered with a container and a clean gas is circulated through the interior thereof.
Still further the present invention is characterized in that the illumination optical system and the detection optical system are covered with a container and a clean gas is circulated through the interiors thereof.
Still further the present invention is characterized in that the illumination optical system, the detection optical system and the stage are placed on an antivibration table and the ultraviolet laser light source is capable of being placed on the antivibration table by being positioned thereon.
Still further the present invention is characterized in that the coherence reduction optical system of the illumination optical system is configured in a unit and the unit is positioned relative to the illumination optical system to enable the installation thereof.